An electric power steering apparatus, which is mounted in vehicles, such as automobiles, is adapted to transmit steering assistant power from an electric motor to a steering mechanism through a speed reducer, based on a steering torque. In an electric power steering apparatus using a worm gear mechanism as the speed reducer, a worm wheel on a steering shaft is engaged with a worm on a driving shaft of the electric motor.
Such a worm wheel has a structure in which an outer circumference of a core made of metal is typically covered with a rim portion made of synthetic resin to reduce collision noise (rattling noise) between meshed teeth due to backlash relative to the worm (see, e.g., Patent Documents 1 to 3).
FIG. 27 illustrates a conventional example of a worm wheel. The worm wheel 100 has a core 100 made of metal and a substantially annular rim portion 201 covering an outer circumferential portion of the core 101. The core 101 is formed in a substantially circular disk shape, and has a center hole 11 into which a shaft is fitted, and an axial end surface formed with a first annular recess 12. A spline 15a is formed on the outer circumferential portion of the core 101.
The rim portion 201 is integrally formed with the core 101 by synthetic resin injection molding. A gear 21 is formed on an outer circumferential portion of the rim portion 201. The rim portion 201 has a first inner circumferential portion 22 fixed to the first annular recess 12 of the core 101 and a second inner circumferential portion 230 fixed to the other axial end surface of the core 101. The first inner circumferential portion 22 extends from one axial end surface of the rim portion 201 to a radially outer region (an outer circumferential wall surface 12a and a radially outer portion 12b of a bottom surface) of the first annular recess 12 and fixed to the outer circumferential wall surface 12a and the radially outer portion 12b of the bottom surface.
Also, the rim portion 201 has a protrusion engaged with a valley of the spline 15a of the core 101. By such an engagement of the protrusion of the rim portion 201 with the valley of the spline 15a of the core 101, the rim portion 201 and the core 101 are coupled to each other to be integrally rotated and thus to allow a torque to be transmitted therebetween.